Fire is part of the lands natural cycle
Fire is an essential natural phenomenon critical to the healthy functioning of many ecosystems. Over two-thirds of the Earth’s land surface experiences periodic fires, and some tropical savannas and grasslands often burn twice a year (1). Many trees have evolved to survive fires, surrounding themselves in protective bark or growing tall above the surrounding flammable grass to keep their leaves above the flames (2). Not all burning is catastrophic. In fact, some plants have made fire a vital part of their life cycle – only releasing or germinating their seeds in the extreme heat of flames to take advantage of much-needed space to grow after the fire (3).
Shifts in fire regimes are upsetting this natural cycle
Changing fire regimes canupset the function of an ecosystem. In savannas, increasing urban areas and agriculture fragment the landscape (4). This stops fire from spreading, and without burning, many species of plants will not reproduce. Preventing natural burning can affect biodiversity and the healthy functioning of some ecosystems (ref). In humid forests – some of the few places where fires do not exist naturally – new fires can devastate a forest that will have no natural adaptation to fire. Increases in burning can be deliberately set by humans to clear land for development (5). In other areas, burning is introduced by invading flammable grasses that grow near newly built roads. The warming climate is lengthening the historically small dry season, especially in the eastern Amazon rainforest, which will likely lead to further burning (6–8).
Surface fires are not the only important fires in the Earth System. Peatland fires have always been around but are occurring more frequently . Increased irrigation has drained many tropical peatlands. These now burn much more easily in dry years, often as part of the El Nino climate phenomenon (when the sea is warmer than usual in the eastern Pacific Ocean). In the Arctic, climate change has driven summer temperatures high enough to set peat-rich permafrost on fire – some burning hot enough to stay alight for years at a time (9).
Fire is tough to model on a computer, and fire’s behaviour under climate change is still very uncertain. Fire regimes are sensitive to small changes in rainfall and its distribution. Compared to temperature, climate model projections for rainfall are less certain. However, using many climate models and advanced statistics, we can find areas where we can be confident in fire risk changes despite uncertainty in modelling fire and future climate. In Indonesia and some Arctic areas, especially in Northern Siberia, climate models suggest dryer conditions will increase burning by the end of the century. These areas contain a lot of peat-rich carbon, potentially exacerbating global warming. Meanwhile, increases in fuel in Eastern Asia, central US, and desert areas of South America will likely increase burning. Less burning is expected in Southern Brazil, Uruguay, and northern Argentina, and along the US east coast under less extreme emission scenarios (but not under more extreme scenarios).
Fire and the climate
Changing fire regimes often effect and have consequences in other parts of the Earth system:
- Carbon: Natural fire regimes are typically in balance with the ecosystem, so carbon released by fire is recaptured by recovering vegetation (10). In changing fire regimes, this carbon balance is upset. This is especially true in humid forests, where tall carbon-rich-woody vegetation is replaced by small, shrubby vegetation. Worse still, some burnt peatland will take thousands of years to recuperate carbon if it were allowed to recover. Most carbon released from all these fires can be in the form of carbon dioxide. However, fires may also be a vital part of the Earth’s methane cycle – which is an even more potent greenhouse gas.
Fire can also affect how plants use their carbon. Protective bark diverts carbon away from plants roots, inner stems or leaves, and many plants store carbon in reserves, so they can resprout and recover rapidly post-burning (2).
- Albedo (how reflective the surface is): Burnt land is darker than natural vegetation. Shifts in vegetation from changing fire regimes may lighten the surface. Especially if burning exposes the lighter soil (11). Lighter soils will reflect more of the sun’s radiation.
- Water cycle: Changes in vegetation distribution and productivity can also affect water evaporated or intercepted by plants. In some parts of the world, the effect needs to be accounted for to accurately predict river flow, most notably for the rivers around some of Australia’s major cities (12).
- Aerosols and cloud formation: Fires release large quantities of aerosol particles into the atmosphere. These particles reflect and absorb solar radiation and can also alter cloud properties (11)
1. D. I. Kelley, thesis, Macquarie University (2014).
2. D. I. Kelley, S. P. Harrison, I. C. Prentice, Improved simulation of fire–vegetation interactions in the Land surface Processes and eXchanges dynamic global vegetation model (LPX-Mv1). Geoscientific Model Development. 7, 2411–2433 (2014).
3. M. J. B. Zeppel, S. P. Harrison, H. D. Adams, D. I. Kelley, G. Li, D. T. Tissue, T. E. Dawson, R. Fensham, B. E. Medlyn, A. Palmer, A. G. West, N. G. McDowell, Drought and resprouting plants. New Phytol. 206, 583–589 (2015).
4. D. I. Kelley, I. Bistinas, R. Whitley, C. Burton, T. R. Marthews, N. Dong, How contemporary bioclimatic and human controls change global fire regimes. Nat. Clim. Chang. 9, 690–696 (2019).
5. D. I. Kelley, C. Burton, C. Huntingford, M. A. J. Brown, R. Whitley, N. Dong, Low meteorological influence found in 2019 Amazonia fires. Biogeosciences. 18, 787–804 (2021).
6. C. Burton, D. I. Kelley, C. D. Jones, Betts R A Cardoso, L. Anderson, South American fires and their impacts on ecosystem increase with continued emissions. Climate Resilience and Sustainability, Royal Meteorological Society (accepted).
7. C. Burton, R. A. Betts, C. D. Jones, Will fire danger be reduced by using Solar Radiation Management to limit global warming to 1.5 C compared to 2.0 C? Geophys. Res. Lett. (2018) (https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2018GL077848).
8. J. A. Marengo, C. M. Souza, K. Thonicke, C. Burton, K. Halladay, R. A. Betts, L. M. Alves, W. R. Soares, Changes in Climate and Land Use Over the Amazon Region: Current and Future Variability and Trends. Front Earth Sci. Chin. 6, 228 (2018).
9. A. Sullivan, M. Castillo, D. Armenteras, H. Safford, D. Kelley, P. Silvia, J. Littell, M. Flannigan, G. Humphrey, D. Ganz, C. Mathison, C. Burton, M. Brown, in Global Wildfires Rapid Response Assessment (RRA), T. Kurvits, E. Baker, A. Sullivan, Eds. (UNEP, submitted).
10. D. I. Kelley, S. P. Harrison, Enhanced Australian carbon sink despite increased wildfire during the 21st century. Environ. Res. Lett. 9, 104015 (2014).
12. A. M. Ukkola, T. F. Keenan, D. I. Kelley, I. C. Prentice, Vegetation plays an important role in mediating future water resources. Environ. Res. Lett. 11, 094022 (2016).